The present invention relates to an exposure apparatus used to expose a pattern on a reticle (or a mask) onto a plate, such as a wafer, in a lithography process for manufacturing a semiconductor device, a liquid crystal device, etc. The present invention is suitable, for example, for a scanning projection exposure apparatus that synchronously scans the photo-mask and the wafer relative to the projection optical system, in projecting a pattern on the mask onto the wafer.
The scanning projection exposure apparatus, such as a step-and-scan system, has been reduced to practice for the purpose of manufacturing semiconductor devices, as well as a cell projection type projection exposure apparatus, such as a stepper. A projection optical system in this type of projection optical system is required to have a resolving power close to the limit, and thus includes one or more units for measuring factors that affect the resolving power, such as the air pressure and the environmental temperature, and for correcting the imaging performance based on the measurement results. The high numerical aperture (“NA”) of the projection optical system, which is set for the improved resolving power, consequently reduces the depth of focus, and thus requires an autofocus mechanism for measuring a focus position on a rough surface of a wafer as a plate (or a position in a direction of the optical axis of the projection optical system), and for positioning the wafer surface at the image surface of the projection optical system based on this measurement result.
However, imaging errors due to reticle's (or mask's) deformations cannot recently become ignorable gradually. For example, if the reticle's pattern surface deforms approximately uniformly toward the projection optical system, an average image-surface position lowers and the defocus occurs if the wafer focus position is the same. When the reticle's pattern surface deforms, positions on the pattern surface may deform in a direction perpendicular to the optical axis of the projection optical system, and the lateral offsets of the pattern causes distortion errors.
Conceivably, the factors of the reticle's deformations are classified into (a) the gravity deformation, (b) the flatness of the reticle pattern surface, and (c) the deformations caused by the flatness of the contact surfaces of the reticle and the reticle holder while the reticle is held by and absorbed on the reticle holder, which deformations include an inclusion of dust. For example, the deformation amount caused by these factors is as large as about 0.5 μm on the reticle, which corresponds to 30 nm on the wafer and cannot become ignored. The deformation amount should be measured at the measurement accuracy of about 0.1 μm. In addition, the reticle's deformation state differs according to reticles and reticle holders in the exposure apparatus. The measurement accuracy of the reticle's deformation amount is maintained when the reticle is actually absorbed on and held by the reticle holder in the projection exposure apparatus.
Accordingly, various methods for measuring the reticle's surface shape (surface position or deformation) have been conventionally proposed. See, for example, Japanese Patent Applications, Publication Nos. 6-36987, 7-272999, 7-86154, 8-264428, 9-180989, 10-214780, and 11-45846.
However, the conventional methods of measuring bows of the reticle use the measuring system and the reticle holder as separate members, bulking the apparatus. In addition, errors in the measuring system, which are hard to eliminate, cause the measurement errors and deteriorate the imaging performance. Moreover, since the measuring system measures a shape of the reticle surface not always at an exposure position, the measurement result does not always provide the intended imaging performance.